The invention is directed toward a support structure for stator vane segments used in a gas turbine engine. The invention is also directed toward an improved stator assembly in a gas turbine engine, which assembly incorporates the support structure.
Second stator assemblies in gas turbine engines usually have the inner radial end of the assembly floating on a seal arrangement on the rotating shaft of the turbine. The outer radial end of the assembly must be fixed to the outer engine casing. This is usually done by a ring-like support structure. However, in fixing the outer end of the second stator assembly to the outer engine casing, thermal expansion of the stator vane segments can cause distortion of the support structure which, in turn, can cause distortion in the outer engine casing. Distortion of the outer engine casing can change blade tip clearances for the blades in adjacent rotor assemblies in the engine which can reduce the efficiency of the engine.
The distortion could be reduced by adequate cooling of the stator vane segments. However, it is difficult to efficiently cool the vane segments when they are fixedly mounted at their outer ends.
Efforts have been made to develop segmented vane support structures which permit thermal expansion of the stator vane segments without causing distortion in the outer engine casing. An example of those efforts is shown in U.S. Pat. No. 5,961,278, issued to Dorais, et al. on Oct. 5, 1999, which is assigned to the assignee of this application. Dorais, et al. describe a cylindrical support structure for use in stator assembly gas turbine engines having an engine casing. The support structure has two outer ring sections between which vane segments of the stator assembly will be mounted and a central ring section by means of which the support structure will be radially located within the engine casing. The rings are joined to form the cylindrical shaped structure by thin, circumferentially spaced-apart spokes extending between each outer ring and the central ring. The spokes are thin enough to flex or distort when the stator vane segments thermally expand, expanding or distorting the outer mounting rings. Thus, the flexible spokes attenuate the distortion transmitted from the outer mounting rings to the central ring and further to the engine casing. The inner ends of the vane segments are mounted between inner engine housings which clamp the vane segments by bolts and nuts to locate them axially and radially.
It is an object of the present invention to provide a support structure for use in a gas turbine engine to mount the outer end of the stator assembly to the engine casing, which support structure permits thermal expansion of the stator vane segments without causing distortion of the engine casing.
It is another object of the present invention to provide a stator assembly which permits thermal expansion of the stator vane segments without causing distortion of the engine casing, and is easily assembled.
In accordance with one aspect of the present invention, there is a support structure provided for supporting vane segments of a stator assembly in a gas turbine engine having an engine casing. The vane segments circumferentially abut to form a stator ring. The support structure comprises means for transmitting a circumferential vane load from each vane segment into the engine casing. The means are disposed between each vane segment and the engine casing. The support structure further includes separated front outer and rear outer rings, which, in cooperation with the engine casing, axially restrain the vane segments between the front outer and rear outer rings, thereby defining an axial position of the vane segments with respect to the engine casing while permitting radial thermal expansion of the vane segments without causing distortion of the engine casing.
In accordance with another aspect of the present invention, there is a method provided for supporting vane segments of a stator assembly in a gas turbine engine and inhibiting transmission of thermal distortion from the vane segments into the engine casing. The method comprises transmitting a circumferential vane load into the engine casing by providing a lug member secured to each vane segment, the lug member being radially slidable in a slot of the engine casing; and defining an axial position of the vane segments within the engine casing using front outer and rear outer rings which are axially separated by the vane segments, the front outer ring being axially restrained by a first annular radial surface of the engine casing and the rear outer ring being axially restrained by a second annular radial surface of the engine casing such that the front outer and the rear outer rings are radially displaceable relative to the engine casing.
It is preferable that the front outer and the rear outer rings axially abut outer edges of the outer platform of each vane segment at opposed ends thereof, respectively. The method preferably further comprises using a retaining ring which is fitted in an inner annular groove of the engine casing, to abut a rear end of the rear outer ring and further to cause a front end of the front outer ring to abut the first annular radial surface of the engine casing.
In accordance with a further aspect of the present invention, there is provided a stator assembly in a gas turbine engine having an engine casing. The stator assembly comprises an inner support ring and a plurality of vane segments circumferentially around the inner support ring and abutting one another to form a stator ring. In cooperation with the engine casing, separated front outer and rear outer rings axially restrain the stator ring with respect to the engine casing, while permitting radial thermal expansion of vane segments without causing distortion of the engine casing. Means are provided for transmitting a circumferential vane load from each vane segment into the engine casing. The means are disposed between each vane segment and the engine casing.
The advantage of the present invention lies in the outer support structure which is constructed from two small rings, one at the front and one at the rear, which clamp the vane segments onto a single piece inner ring and the vane segments themselves have lug members for positioning the assembly and reacting the torque loading. In this arrangement, the angular positioning of each vane segment within the engine casing is controlled by one set of lug members and slot interfaces only and the circumferential vane loading from each individual segment is transmitted by its own lug member into the engine casing, which provides an even loading of the lug members in the structure. With the separated outer rings, it is possible to assemble the vane segments onto the single piece inner support ring that requires no bolted features, rivets, welds or mating parts to retain the segments, since the segments are retained by the two outer rings which are axially restrained within the engine casing by a retaining ring.
Other advantages and features of the present invention will be better understood with reference to a preferred embodiment of the present invention described hereinafter.